Impact of ionizers on prevention of airborne infection in classroom

Infectious diseases (e.g., coronavirus disease 2019) dramatically impact human life, economy and social development. Exploring the low-cost and energy-saving approaches is essential in removing infectious virus particles from indoors, such as in classrooms. The application of air purification devices, such as negative ion generators (ionizers), gains popularity because of the favorable removal capacity for particles and the low operation cost. However, small and portable ionizers have potential disadvantages in the removal efficiency owing to the limited horizontal diffusion of negative ions. This study aims to investigate the layout strategy (number and location) of ionizers based on the energy-efficient natural ventilation in the classroom to improve removal efficiency (negative ions to particles) and decrease infection risk. Three infected students were considered in the classroom. The simulations of negative ion and particle concentrations were performed and validated by the experiment. Results showed that as the number of ionizers was 4 and 5, the removal performance was largely improved by combining ionizer with natural ventilation. Compared with the scenario without an ionizer, the scenario with 5 ionizers largely increased the average removal efficiency from around 20% to 85% and decreased the average infection risk by 23%. The setup with 5 ionizers placed upstream of the classroom was determined as the optimal layout strategy, particularly when the location and number of the infected students were unknown. This work can provide a guideline for applying ionizers to public buildings when natural ventilation is used. Electronic Supplementary Material (ESM) the Appendix is available in the online version of this article at 10.1007/s12273-022-0959-z.

shows the photo of experimental setup in the indoor chamber, including air conditioner, thermal anemometer, air quality meter, air ion counter, negative ion generator (ionizer) and particle release device. Figure A2 shows the diagrams of the on-site measurement of ion generation rate using an air ion counter (Alphalab Inc AICZX21), and the ionizer device used in this work. The unit of this air ion counter is 1 million ions per cm 3 . The ion generation rate of an ionizer was measured with the measurement distance of 0.1 m between the air ion counter and ionizer. The measurement period was set as 30 min, and the average value of negative ion generation rate was measured as 50 million #/cm 3 , and positive ion generation was neglected. The ion counter was also used to record the negative ion concentrations at the monitoring locations at different heights in the indoor chamber.

Appendix B Influence of additional locations of ionizer on particle distribution, removal efficiency, and infection risk
The layout strategy of ionizers located upstream of the classroom (see Figure 5) was investigated, by analyzing the negative ion and particle distributions, removal efficiency, and infection risk. The removal performance of ionizers could be deteriorated because of the limited diffusion capacity of negative ions when the distance from the ionizers increased. The additional layout strategies of ionizers (in the middle area and downstream of the classroom) were considered to analyze the influence of ionizer layouts further. Figure B1 presents the additional locations of ionizers A1-A5 and A6-A10 with the infected student at S1, S2, and S3. The overview of the simulation cases under different scenarios (G and H) of infected students and ionizers in the middle area and downstream of the classroom is listed in Table B1.   Figure B2 shows the particle distribution at the plane of Z = 1.1 m when the infected student was at S1, S2, and S3, under the additional locations of the ionizers of A1-A5 and A6-A10. When 5 ionizers were installed in the middle area of the classroom and the infected student was at S1, S2, and S3, the coverage percentage of particles could reach 17.5%, 12.5%, and 25% in the desk area, respectively. The coverage percentages of particles were 27.5%, 25%, and 15% in the desk area when the ionizers were installed downstream of the classroom and the infected student was at S1, S2, and S3, respectively. The removal performance was improved when the distance away from the ionizers decreased. The finding was consistent with Figures 9-11.  Figure B3 shows the removal efficiency of ionizers (number is 5) and the infection risk when the infected student was at S1 + S2 + S3 and the additional ionizers were in the middle area and downstream of the classroom. When the ionizer location was away from the upstream, the removal efficiency decreased from 85.4% (see Figure 12) to 77.3% and 70.9%. The infection risk (with infected students at S1 + S2 + S3) when the ionizers were located downstream of the classroom largely increased by 6.1% compared with that when the ionizers were located upstream. From the perspective of removal efficiency and infection risk, placing ionizers upstream of the classroom was more favorable than placing them at the additional locations, as shown in Figure B1.

Appendix C Particle distribution, removal efficiency, and infection risk without natural ventilation when independently using ionizers
To analyze the influence of independently using ionizers, the layouts of ionizers (upstream of the classroom, as shown in Figure 5) were also considered in the classroom without natural ventilation. Table C1 lists the overview of the simulation cases under different scenarios (I, J, K, L and M) of infected students and ionizers upstream of the classroom without natural ventilation. Figure C1 shows the particle distribution at the plane of Z = 1.1 m with the infected student at S1, S2, and S3 and the ionizers (with number of 1-5) upstream of the classroom without natural ventilation. Figure C2 shows the removal efficiency and infection risk of independently using ionizers under different scenarios of ionizer layouts and infected students (S1, S2, S3, and S1 + S2 + S3). Tables C2 and C3 show the significant difference analysis results based on p-values under different numbers of ionizers. Compared with the removal efficiencies when ionizers and natural ventilation were combined (in Figure 12), the removal efficiencies of ionizers used independently were reduced with a maximum percentage of 48% (with the ionizer number of 5 and infected student at S1). The infection risk of independently using ionizers was reduced when the number of ionizers was increased. However, the minimum infection risk with ionizers used independently was still larger than that with the natural ventilation used independently. Thus, combining the ionizers and natural ventilation is necessary.

Fig. C1
Contour of relative particle concentration (C/Cref) at the plane of Z = 1.1 m under infected student of S1, S2, and S3 and different scenarios of ionizers upstream of the classroom without natural ventilation

Fig. C2
Removal efficiency and infection risk of independently using ionizers under different numbers of ionizers (1-5) upstream of the classroom and infected students of S1, S2, S3, and S1 + S2 + S3  Appendix D Removal efficiency with two infected students (S1 + S2, S1 + S3, and S2 + S3) and different numbers of ionizers The removal efficiency with two infected students (S1 + S2, S1 + S3, and S2 + S3) and different numbers of ionizers (0-5) is demonstrated in Figure D1. Table D1 displays the significant difference analysis results based on p-values under different numbers of ionizers (0-5). The removal efficiency was obtained based on the particle concentrations in the breathing area of Z ≤ 1.1 m. The removal efficiency increased linearly as the ionizer number increased. When the number of ionizer was 0 and 1, the average removal efficiencies were below 20% with two infected students. The average removal efficiency was increased as the number of ionizers increased to 2, 3, 4, and 5. Five ionizers could contribute to the comprehensive removal of particles, particularly when the location of the infected student changed.